Testing ignition coil

[dmal...@umich.edu]

I have an 85 Saab 900 that died suddenly, no coughing, nothing.
I did the test where you hold the secondary coil wire and hope not
to get shcoked. No spark. So I wanted to test the coil.
My Haynes manual said to check the resistance in the primary and
secondary circuits. The primary was in spec, but the secondary
was not. It's supposed to be 55k to 85k ohms. But I got 7k ohms.
I tested a new coil at the store and got 7k ohms.
I subsequently found a short in the electronic control unit and the
car started and ran fine (so far!).
My question is: can anybody tell me if this resistance test is a
good one for a coil? and, if a coil is bad, should I see more or
less resistance? and, do coils go bad all at once, or intermitantly
and then fail?
Thanks!
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[Don Vanco]

I'm no expert here but I believe a test for DC resistance can be a
fairly good guage - but an inductive test would be more accurate. If
the spec is ~10 times higher than what you're reading & yet a new coil
reads the same I'd suspect that there is some type of ballast resistor
in the picture or that you are reading across something unseen.
When a coil fails it usually just opens - but I have seen them start to
"melt" and short through windings slowly - but the increase in
heat/current this causes usually results in an avalanche effect that
quickly opens the coil (and can provide the occasional spectacular light
show....)
Don

[Greg Ebert]

dmal...@umich.edu wrote:
>
[...]

> My question is: can anybody tell me if this resistance test is a
> good one for a coil? and, if a coil is bad, should I see more or
> less resistance? and, do coils go bad all at once, or intermitantly
> and then fail?
> Thanks!

A resistance test will tell you if your coil is open, or if there
is a gross short. In my opinion, the probability of a gross short
is very small.

A resistance test will NOT reveal insulation-breakdown problems;
breakdown can cause intermittent missing, weak-spark, or even no
spark.

[Chip McCoy]

dmal...@umich.edu wrote:
>I have an 85 Saab 900 that died suddenly, no coughing, nothing.
>I did the test where you hold the secondary coil wire and hope not
>to get shcoked. No spark. So I wanted to test the coil.
>My Haynes manual said to check the resistance in the primary and
>secondary circuits. The primary was in spec, but the secondary
>was not. It's supposed to be 55k to 85k ohms. But I got 7k ohms.
>I tested a new coil at the store and got 7k ohms.
>I subsequently found a short in the electronic control unit and the
>car started and ran fine (so far!).

>My question is: can anybody tell me if this resistance test is a
>good one for a coil? and, if a coil is bad, should I see more or
>less resistance? and, do coils go bad all at once, or intermitantly
>and then fail?
>Thanks!

>-------------------==== Posted via Deja News ====-----------------------
> http://www.dejanews.com/ Search, Read, Post to Usenet

Yes, testing the resistance of a coil is a good test as long as you have
the knowledge to interpret it correctly. A coil can fail with high or low
resistance on either the Primary or Secondary side. Let me illustrate, if
the primary resistance is too low it will allow excess current to flow
through whatever is the controlling device; i.e. set of breaker points,
ignition solid state module,etc. Many times when this occurs the
technician replaces the "Bad" module without checking the Primary
resistance. The customer ends up going down the road with the new module
and comes right back a couple of days later, because the low Primary
resistance caused another module failure. A high Primary resistance will
cause insufficient Primary current which will directly affect the
Secondary voltage. This could give the complaint of good idle operation
but engine cuts out and misses at high loads or highway speeds. A Low
secondary resistance can indicate shorted turns which will cause
deficient spark output and a high secondary can also cause engine missing
and cutout at load.
In my experience I've seen coils fail right now and yet at other times
it's a gradual situation.
Good Luck

Charles "Chip" McCoy
Supervisor of Advanced Service Engineering
Delco Electronics - Kokomo, Indiana. USA